Recent advances in Silicon Photonics technology, enabled by the technology's compatibility with complementary-metal-oxide-semiconductor (CMOS) production techniques, have made Photonic Systems on a Chip (PSoC) a reality. PSoCs may comprise a large number of application specific photonic circuits on a chip of only a few squared millimetres. This means PSoCs may be used to produce high scale optical switching devices, high speed multi-wavelength transceivers or other types of optical processing devices.
However, PSoCs have optical losses, which increase in dependence on the number of photonic circuits integrated in a chip. In many applications such as data centers or radio access networks optical interconnect interfaces coupled to PSoCs do not have a large enough power budget to tolerate such optical losses. Although an Erdium Doped Fibre Amplifier (EDFA) could be used to increase the power of an optical signal output by a PSoC, this solution is not viable due to cost.
Therefore, a substantially loss-less PSoC is desirable. However, since silicon material is incapable of generating light, a light amplification element in the form of a semiconductor optical amplifier (SOA), made from III-V materials, would need to be integrated into the silicon photonics chip. Similarly, if it is desired to produce a PSoC with complex multi-channel optical transceivers including several lasers, it is necessary to integrate lasers in the form of dies with III-V active layers into the silicon chip.
Various techniques have been researched for integrating SOAs/lasers into PSoCs, for example as detailed in Zhiping Zhou, Bing Yin and Jourgen Michel “On-chip light sources for silicon photonics” Light: Science and Applications (2015) 4, e358; doi: 10.1038/lsa.2015.131, 2015. A feasible technique is based on flip-chip bonding of 111-V dies (with SOA and/or laser functions) on the silicon substrate. In this technique an interface of the 111-V die is butt coupled in front of a silicon waveguide. However, the optical beam spot size of silicon waveguides (normally of a few hundred nanometer) and the optical beam spot size of the 111-V dies are very different. Thus, in order to achieve a low coupling loss, optical beam Spot Size Convertor (SSC) circuits need to be used.
There are a number of existing SSC circuits. A first type of SSC is described in Shimizu T, Hatori N, Okano M, Ishizaka M, Urino Y et al “High density hybrid integrated light source with a laser diode array on a silicon optical waveguide platform for interchip optical interconnection” 8th IEEE International Conference on Group IV Photonics, IEEE 2011 page 181-183; and in Hirohito Yamada “Analysis of Optical Coupling for SOI waveguides” PIERS Online Vol. 6, No. 2, 2010. This first type of SSC 100 is illustrated in FIG. 1.
The SSC 100 comprises a silicon (Si) optical waveguide 105 surrounded with a silicon enriched oxide (SiOx:) optical waveguide core layer 110. The silicon optical waveguide 105 and silicon enriched oxide optical waveguide core 110 are surrounded by a silica cladding layer 115 and are on top of a buried oxide (BOX) layer 116, which is arranged on a silicon (Si) substrate 120. The silicon optical waveguide 105 has an inverse taper, whereby the width of the silicon optical waveguide 105 gradually increases, along a portion of its length extending from a first end 125 of the silicon optical waveguide 105. Light input, for example from a III-V die, is confined inside the silica structure and during its propagation the light changes its mode shape so as to fit into the silicon optical waveguide 105. In this way, the light output from the SSC 100 has a different optical beam spot size from the light input into the SSC 100.
However, this SSC 100 has the disadvantage that its length is some hundreds of micrometres (μm), which means that the SSC 100 is too large for use in high integration scale PSoCs. Furthermore, the coupling loss of this SSC 100 with perfect input/output alignment exceeds 2 dB while with a misalignment of +−1 μm (which corresponds to the alignment accuracy of a commercial flip-chip machine) a total coupling loss of 3 dB has been measured.
A second type of SSC is described by Nobuaki Hatori et al in “A hybrid integrated light source on a silicon platform using a trident spot size convertor” IEEE JLT, Vol. 32, N.7 (2014) and illustrated in FIG. 2. This SSC 200 comprises three tapered silicon waveguides 205 arranged in the shape of a trident fork. These silicon waveguides 205 are surrounded by a silica cladding layer 210 on top of a buried oxide (BOX) layer 211, which is arranged on a silicon substrate 215. As in the first type of SSC 100, light input into the SSC 200, for example from a III-V chip, is confined inside the silica structure and during propagation the light changes its mode shape so as to fit into the silicon waveguides 205. In this way, the light output from the SSC 200 has a different optical beam spot size from the light input into the SSC 200.
However, this second type of SSC 200 has similar disadvantages to the first type of SSC 100 described above. The length of the SSC 200 in FIG. 2 is still about 150 μm. Furthermore, the SSC 200 has similar coupling loss characteristics to that of the SSC 100.
The Applicant has appreciated that it would be desirable to provide an optical beam spot size convertor having a smaller size than the above-described SSCs without increasing and preferably decreasing coupling loss and/or misalignment tolerance. The Applicant has further appreciated that it would be desirable to provide an optical beam spot size convertor which is simple to mass-produce, at low cost. Advantageously, such an optical beam spot size convertor may be used to increase the density of photonic integrated circuits in a photonic systems chip.